The present invention relates to a zinc- or zinc alloy-hot-dipped, unpainted steel sheet having excellent black tarnish resistance and to a process for producing such steel sheets.
Recently, with the expanding demands and the diversification of applications, there are growing needs for further improvements of hot-dipped galvanized steel sheets in performance characteristics of, for example, appearance, workability, chemical treatability (treatability before painting), and corrosion resistance. For improving the workability and chemical treatability, the so-called "zero-spangle treatment" is practiced, which treatment is carried out by spraying atomized water against the surface of a molten zinc layer after hot dipping and before the solidification of the layer, to make fine zinc crystals. Further the treated sheet is subjected to skin-pass rolling (usually 1-3% draft) for improving the appearance or surface microscopic geometry (surface smoothness).
However, the skin-pass rolling, when followed by chromating for improving the corrosion resistance and organic coat adhesion, is often caused the trouble of discoloring the plating surface to blue-brown-black (hereinafter the discoloring is referred to as black tarnishing) during relatively short time storage or during transport.
For improving the corrosion resistance, techniques of alloying zinc by adding Al, Mg, Ti, or a rare earth element (La or Ce) to a molten Zn bath have been studied inside and outside this country and some of them were already put into practical operation.
However, when these zinc alloy-hot-dipped steel sheets were chromated, black tarnish was observed on the plating surface during storage or transport similarly in case of zero-spangle treated galvanized steel sheets (skin-pass sheets).
When these zinc alloy-hot-dipped steel sheets are skin-pass-rolled (draft 1-3%), black tarnish will appear thereon in shorter time. Also on usual hot-dipped galvanized steel sheets (non-skin-pass sheets), black tarnish due to the spangle is occasionally observed in spotty form during long-term storage, but the black tarnish on the above-mentioned zero-spangle treated skin-pass sheets and alloy plated sheets is characterized by developing for a very short period of time. In addition, the black tarnish is more liable to develop when these sheets are stored in stacked form after cutting into pieces or in coiled form than when these are left standing in bared form in a room. As regards the effect of environments, the black tarnish is liable to develop in high temperature and humidity surroundings.
The present inventors detected a thicker layer (estimated at hundreds-thousands .ANG.) of zinc hydroxide-zinc carbonate mixture on the plating surface where black tarnish had developed than on the surface where black tarnish did not yet develop. From this result and the above information, the mechanism of the black tarnish development is presumed as follows: on the surface of the zinc alloy-hot-dipped steel sheet, Al or Mg added for alloying is present in the form of metal or oxide and forms local cells jointly with Zn, thereby causing the electrochemical corrosion (reaction of metallic zinc with moisture and chromic acid) to proceed abnormally quickly under corrosive environmental conditions mediated by the chromate coating layer which inherently contains moisture and is liable to absorb moisture. The black appearance of the plating surface may be contributed to by the light interference due to the presence of a thin zinc hydroxide-zinc carbonate layer on fine depressions and projections formed by the corrosion of the Zn layer.
The reason why the skin-pass-rolled sheet becomes liable to undergo black tarnishing is conceivably because strain and fresh Zn surfaces are produced in the plating layer by the rolling, which develops more chemically active local cells accelerating the corrosion. For the purpose of preventing the plating surface from being black tarnished, there has been proposed after-treatment with non-chromate chemicals, or zinc electroplating. For instance, a method of coating with a water-soluble resin has been proposed as the non-chromate after-treatment method. However, as can be assumed from the above-mentioned mechanism of black tarnish development, a thin resin coating cannot shut out the penetrating moisture, thus allowing the black tarnishing.
Recently, Japanese Patent Application Laid-Open No. 114695/82 discloses a process for producing galvanized steel sheets superior in resistance to black tarnishing and resistance to intergranular corrosion. This technique comprises plating steel sheets with zinc or a zinc alloy by hot dipping, and treating the plated sheets with an alkali metal carbonate solution, followed by electroplating the treated sheets with zinc. According to this patent application, it is said that oxides of Zn, Al, Fe, and Si formed on the hot-dipped galvanized surface are completely dissolved by the treatment with an alkali metal carbonate solution and this permits the black tarnishing and the intergranular corrosion to be prevented even when the subsequent zinc electroplating gave a thin film.
The above-mentioned technique, however, requires two additional operation steps of alkali cleaning and zinc electroplating, and thus high investment cost for equipment and complicated management of the plating bath being inevitable.
Other examples of prior art include the techniques disclosed in Japanese Patent Application Laid-Open No. 152834/77 entitled "Method for Pretreatment of Hot-Dip Galvanize Steel Sheet before Painting", Japanese Patent Application Laid-Open No. 115624/78 entitled "Method for Conditioning Galvanized Steel Sheet Surface", and Japanese Patent Publication No. 26434/83 entitled "Method for Zinc Surface Treatment". These known techniques, however, deal with only the adhesion of organic coating materials and the corrosion resistance of the coating, as problems to be solved. These Japanese Patent Application Laid-Open Nos. 152834/77 and 26434/83 disclose the method of removing the oxide film formed on galvanized steel sheets and dissolving the zinc surface at the same time, by means of a strong acid solution containing Co, Ni, or Fe metal ions, to substitute and deposit the metal. It is widely known that the surfaces of galvanized steel sheets when etched with an acid shows a white, gray, or black appearance according to spangle patterns. The etching followed by the metal substitution-deposition intensifies the black tarnish. The Japanese Patent Application Laid-Open No. 115642/78 discloses the method of simultaneously conducting the formation of a phosphate coat and the removal of the oxide film, dissolution of zinc, and substitution-deposition of a metal. This phosphate coat is similar in coating weight to those obtained by the usual phosphating, which are well known to exhibit a gray or gray-black color. Thus, steel sheets according to these three known methods each exhibits a color (gray, brown, or black) characteristic of the coat before chromating, in other words, black tarnishing of the sheets has already occurred before chromating. The subsequent chromating will add a yellow color to the above color, resulting in a gray-black-yellow mixed color. However, the three methods of the above-mentioned three Japanese documents do not intend to remove a color or avoid the black tarnishing since the sheets after chromating are painted.